1. Cut the puff pastry sheets into 5-inch (13cm) squares. Set aside in the refrigerator. Pre-heat your oven to 400°F(200°C).2. For the bechamel sauce: Melt butter over medium heat in a saucepan. Add flour and salt; cook, stirring frequently, until the mixture becomes pale golden with a slightly nutty aroma.3. Pour warm milk in 2 steps, stirring constantly with wooden spoon and whisk until smooth. Cook the mixture, stirring constantly along the bottom of the saucepan, until boiling, about 7 minutes.4. Reduce heat to low. Simmer gently until sauce thickens, about 10 minutes. If you still have lumps, use your immersion blender!5. Stir in spinach and cook for 1 minute. Add ham and cheese and stir well. Set aside.6. Arrange the puff pastry squares on a baking sheet lined with parchment paper. Dollop 1 or 2 tablespoons of the bechamel mixture, top with grated cheese and close the pockets by folding the corners on top.7. Make an egg wash using an egg and 1 coffee spoon of cold water. Brush the puff pastry and bake in the oven for 20 minutes, until puffed and golden. Serve immediately with a crisp salad.